104 



Classification. — Mr. Darwin has divided Coral reefs into 

 I. Atolls, or Lagoon Islands, 

 II. Barrier, or Encircling Reefs, 

 III. FRiNGHNa, or Shore Eeefs. 



I. Atolls, so named by their inhabitants, consist of a wall of 

 Coral rock rising in the ocean from a considerable depth, and 

 returning into itself, so as to form a calcareous ring, more or 

 less complete, having a sheet of shallow water, or lagoon, within 

 the circle. The wall is breached in general, in one or more 

 places, and when sufficiently wide and deep to admit a vessel, 

 it may form a convenient harbour. The outer side of the reef 

 slopes away at an angle of 45° or more, to a depth of from two 

 to five hundred fathoms, whilst the inner side incUnes gently 

 towards the middle of the lagoon, forming a saucer-shaped 

 cavity, having a depth of water from one to fifty fathoms. The 

 annexed sketch of Whitsunday Island, in the Sovith Pacific, 

 taken from Admiral Beechey's work, affords a faint idea of the 

 general aspect of one of these Atolls, or lagoon islands. 



Fig. 1. — "Whitsunday Island. 



M. Darwin remarks, * that "this island is one of the smallest 

 in size, one mile and a half long, and has its narrow islets 

 united together in a ring of land, which is comparatively rare, 

 so that it does not represent the singular aspect of one of these 

 islands. The immensity of the ocean, the fury of the breakers, 

 contrasted with the lowness of the land and the smoothness of 

 the bright green water within the lagoon, can hardly be imagined 

 without having been seen." 



* Naturalists' Voyage, p. 466. 



